1. Field of the Invention
The present invention relates to front and rear projection screens of the type comprising a plurality of rectilinear rows of optical microelements, each of which is specially contoured to distribute image flux so as to produce substantially constant luminance throughout a predefined solid audience angle. More particularly, the present invention relates to improvements in the aesthetic appearance of such screens. Additionally, the present invention relates to apparatus for fabricating the screens.
2. Description of the Prior Art
In the above-referenced U.S. Pat. No. 3,754,813 radiation redistributive devices such as front and rear projection screens are disclosed having a light distributing surface which comprises a plurality of rectilinear grooves. The depth of each of the grooves undulates in a periodic manner along the groove length in accordance with a predetermined waveform to define a row of substantially uniformly sized optical microelements which produce substantially uniform radiance throughout a predefined region of utility. U.S. Pat. No. 3,765,281 discloses an apparatus and a method for fabricating such a projection screen. Briefly, the screen is fabricated by employing the cutting stylus of a sound recording head as a tool for cutting the light distributing surface of the screen and by modulating the cutting position of the stylus with an electrical signal of a predefined waveform. Ideally, the undulation representing the depth profile of each groove should be perfectly in phase with that of all other grooves, since slight changes in phase relationship between optical microelements give rise to a surface having a streaky appearance. However, since the grooves are necessarily cut in a sequential manner, the work piece from which the projection screen is fabricated being moved past the cutting stylus in a series of equally spaced parallel traverses, it is exceptionally difficult, due to minute variations in the velocity of the work piece, minute variations in the frequency of the signal used to modulate the cutting stylus, and the electro-mechanical transducer delays associated with the mounting structure of the cutting stylus, to maintain the ideal phase relationship from one groove to another. Commonly, the screen surface exhibits random streaks of light and dark areas running parallel to the grooves which are unpleasing to the eye. Also, the array of small, uniformly-sized microelements may give rise to diffraction fringes which, in turn, may cause a microelement to appear lighter or darker than it should, depending on the specific point in the audience space from which it is viewed, or even introduce color where there should be none.
To improve the aesthetic appearance of the screens, U.S. Pat. Nos. 3,754,811 and 3,788,171 disclose that the signal used to modulate the cutting position of the stylus should be intentionally randomized in frequency so as to produce optical microelements of random size but of similar image flux-distributing contour. While showing marked improvement in aesthetic appearance over similar screens comprised of substantially equally sized microelements, such screens still exhibit some diffraction effects when irradiated (e.g. color banding) which are somewhat unpleasant to the eye. These diffraction effects are noticeable to the viewer because the frequency of the randomized stylus control waveform is changed slowly in order to maintain substantial continuity of waveform slope and therefore light redistribution profile for adjacent microelements. Since the distance from a point on one microelement contributing to the appearance of a particular color to corresponding points on adjacent microelements is substantially equal, the color banding problem is not eliminated.
U.S. Pat. No. 3,809,457 discloses an improved screen in which the microelements are randomly sized, at least in one transverse direction, to the extent that the slope of a particular microelement's contour at any point along a row is not predictable from a knowledge of the slope of other microelement contours along the row. The distribution of such randomly sized microelements is predictable, however, to the extent that, on the average, all reasonably small areas on the screen (i.e., an area which is small compared to the entire area of the screen itself but nevertheless is large compared to the transverse structure defining each microelement) redirect normally incident light into solid audience angles of the same size and shape. Such a screen has improved aesthetic qualities because the random structure causes all small areas on the screen to appear uniformly bright to the viewer. U.S. Pat. No. 3,809,457 also discloses a method and apparatus for fabricating such a screen. Briefly, such a screen is fabricated by applying to the cutting stylus an electrical signal having a waveform which is random at least to the extent that the slope of such waveform is, at any point unpredictable (within a predefined range of slope values) from the knowledge of the slope at other points on the waveform, but is predictable to the extent that the slope probability density profile (i.e., its frequency distribution of slope values) over the irradiated screen area has substantially the same shape as the radiation-redistribution profile desired from that area.
While screens produced according to teachings of U.S. Pat. No. 3,809,457 exhibit an aesthetically pleasing surface when viewed from within the intended audience viewing angle, complex circuitry is needed to produce the required randomly varied cutting stylus signal. The reason why such circuitry is necessarily complex is briefly explained as follows. Random variation of the frequency of the cutting signal means that the signal generator must be frequency modulated by a noise signal. It is simple enough to build or buy a generator of an electrical noise signal and to feed it to any one of several commercial "function generators" whose frequency can be controlled by an externally supplied input voltage. However, if the frequency of the cutting signal is altered while its amplitude remains constant, the proportions of the microelements change and, therefore, so does the angle over which they spread light. To keep the larger and smaller microelements all with the same optical power, it is necessary to change the amplitude of the cutting signal in such a way that the amplitude is always proportional to the wavelength of the cut, which means inversely proportional to the cutting signal frequency. All this implies that the cutting signal amplitude must be under control of the same noise modulating signal that frequency modulates the generator.